The present document is based on published International Patent Application No. PCT WO 01/05708, the entire contents of which being incorporated herein by reference.
1. Field of the Invention
The present invention is related to a process and a plant for purification of a liquid, said liquid being contaminated by gas, other liquids and/or solid materials.
2. Discussion of the Background
It is known to use means like centrifuges, hydrocyclones, and flotation systems to treat waste water containing oil or other contaminants. Centrifuges are big, heavy, energy demanding and expensive especially for treating flows of above 100 m3/h. Hydrocyclones have been used extensively for treating oil containing waste waters, and they have proven reliable under steady conditions. However, hydrocyclones have their disadvantages as they can not give very low outlet contaminant values, e.g. usually not under 25 mg/l dispersed oil, and if larger capacities are required a plurality of hydrocyclones is needed in parallel. A degassing tank is often used downstream of the hydrocyclones. Hydrocyclones geometrical shape leads to high production costs. A common flotation system requires big space, and usually it is not separately a high efficiency device.
By flotation a substance, e.g. in the form of droplets or particles, is removed from another directly or indirectly by means of buoyancy forces, such as when gas bubbles are supplied to one of the substances, the gas bubbles thereby attaching to the substance to be removed, by natural flotation and gas or air flotation respectively.
Flotation is performed as dissolved gas flotation when the entire or part of the liquid flow is pressurized in such a way that a desired amount of gas is dissolved in the liquid, whereafter the pressure is reduced and the gas releases as gas bubbles, or as induced gas flotation where gas is supplied directly to the liquid flow in such a way that gas bubbles are created.
To achieve a high efficiency, a complete separation process comprises destabilisation, precipitation and flocculation, addition of gas and flotation, respectively. There are many different system solutions for these unit processes in use.
By dissolved gas flotation the complete or a part of the input flow or output flow is pressurized to add gas. Separation is performed in a tank whereby the main portion of the contaminants in the form of particles/droplets float to the surface of the liquid or to the upper portion of the tank. Therefrom the contaminants are skimmed, drained or pumped away. A small fraction of the contaminants sink to the bottom where it is scraped and pumped away.
WO 96/12678 describes a process for purification of a liquid, where the liquid is contaminated by other liquids or solid materials, for example removing oil from oil contaminated water. The process comprises a flocculation device, a bubble generator, and a flotation device or a sedimentation device. The flocculation device comprises a pipe loop of one or more vertically arranged pipe elements with built-in agitators to create turbulence and plug-type flow through the loop. Each pipe element has a built-in agitator comprising a shaft, which extends through the element, provided with propellers. A motor drives the agitator. The bubble- and floc-containing liquid passes through a static mixer and a diffusor arranged between the flocculation device and the flotation device; The diffusor is connected to the flotation inlet at the lower part of the flotation device. The diffusor results in a speed reduction of the liquid before it enters a flotation chamber of the flotation device. As the static mixer generates bubbles in the liquid from added gas, the process is based on induced gas flotation. This process has several disadvantages over the present invention as external energy is used to build up flocs with agitators and gas is added rather than using existing gas. Another disadvantage is that large shearing forces in the mixer can destroy the already built-up flocs.
The known processes have further disadvantages such as floc destruction by large shearing forces in pumps, cyclone inlets and valves downstream of the flocculation process. Known processes based on recirculation have the disadvantage that the recirculation as such increases the loading on the separation process and the requirement of a recirculation pump. This leads to larger tanks and consumption of external energy by pumps. Agitators in flocculation processes consume energy and give high operating expenses.
In view of the above mentioned disadvantages associated with prior art processes and systems, the object of the present invention is to provide a process and a plant where said disadvantages are avoided or eliminated. Another object of the present invention is to provide a process and a plant for purification of a liquid by only using the energy of the liquid flow. These objects are achieved by a process and a plant as defined in the appended claims. Further advantages of the process and plant according to the present invention will be evident in the following description.